Chemogenetic inhibition of cells in the paramedian midbrain tegmentum increases locomotor activity in rats

Pronounced hyperactivity can be produced by lesions or pharmacological inhibition of cells in the median raphe nucleus (MR) located in the paramedian midbrain tegmentum. In the current study we examined whether a similar effect can be seen after chemogenetic inhibition of cells in this region using the DREADD (Designer Receptors Exclusively Activated by Designer Drugs) approach. We found that the DREADD ligand clozapine-N-oxide (CNO) increased locomotor activity in animals expressing the inhibitory DREADD hM4Di, but not those injected with a control virus in the MR. The effect was of rapid onset and short duration and persisted for at least four months after virus injections. Histological examination of the brains indicated that labeled fibers followed the known projection patterns of the MR to a variety of forebrain and midbrain structures. These findings confirm the role of the MR region in the control of locomotion and suggest that the DREADD technique may be a useful approach to the study of the functional architecture of this complex area. Methodological and interpretive aspects of DREADD studies are discussed.

Manipulation of GABA in the ventral pallidum, but not the nucleus accumbens, induces intense, preferential, fat consumption in rats

Injections of the GABAA antagonist bicuculline into the medial ventral pallidum (VPm) induce marked increases in food intake, but nothing is known about the way in which these injections alter the distribution of intake in a macronutrient selection situation. We investigated this topic by adapting rats to a diet containing independent sources of protein, carbohydrate and fat, and then examining the effects of intra-VPm bicuculline on diet selection. Under these conditions, bicuculline produced a massive, preferential increase in fat intake with subjects consuming a mean of 97% of their calories from fat. Furthermore, all treated subjects ate fat before any other macronutrient, suggesting that the animals' behavior was directed selectively toward this dietary component even before consumption had begun. Similar effects were not observed following food deprivation, which exerted its largest effect on carbohydrate intake. To compare the intra-VPm bicuculline response to that seen after activation of GABA receptors in the nucleus accumbens shell (AcbSh), a major source of projections to the VPm, we conducted similar experiments with intra-AcbSh injections of muscimol and baclofen. These injections also enhanced food intake, but did not reproduce the selective preference for fat seen after intra-VPm bicuculline. These experiments provide the first demonstration of preferential enhancement of fat intake following manipulations of a nonpeptide neurotransmitter. Since mean intakes of fat under baseline conditions and after deprivation tended to be lower than those of carbohydrates, it seems unlikely that the effects of intra-VPm bicuculline are related to the intrinsic "rewarding" properties of fat, but might rather reflect the induction of a state of "fat craving."

Dopamine is differentially involved in the locomotor hyperactivity produced by manipulations of opioid, GABA and glutamate receptors in the median raphe nucleus

The median raphe nucleus (MR) has been shown to exert a powerful influence on behavioral arousal and marked locomotor hyperactivity can be produced by intra-MR injections of a variety of drugs including GABAA and GABAB agonists, excitatory amino acid antagonists, and μ- and δ-opioid agonists. Other studies have indicated that the MR exerts an inhibitory influence on ascending dopamine systems, suggesting that MR induced alterations in activity may be mediated through changes in dopaminergic transmission. In the present study, we explored this possibility by examining whether systemic administration of the preferential D2 dopamine antagonist haloperidol is able to antagonize the hyperactivity produced by intra-MR injections of various drugs. We found that haloperidol completely blocked the locomotor response to intra-MR injections of the μ-opioid receptor agonist DAMGO and the δ-opioid receptor agonist DPDPE. In marked contrast, at doses which abolished the locomotor response to systemic amphetamine, haloperidol had no effect on the hyperactivity induced by intra-MR injections of GABAA agonist muscimol, the GABAB agonist baclofen, or the kainate/quisqualate antagonist pBB-PZDA, even though it suppressed baseline activity in these same animals. These results indicate that there must be at least two mechanisms capable of influencing behavioral arousal within the MR region, one of which is dependent on D2 dopamine receptors and the other is not.

Lateral hypothalamic involvement in feeding elicited from the ventral pallidum

Intense feeding can be elicited by injections of the GABA(A) receptor antagonist bicuculline into the medial ventral pallidum (VPm), a basal forebrain structure anatomically interposed between two other feeding-related brain regions, the nucleus accumbens shell and the lateral hypothalamus (LH). To determine whether the VPm effects changes in feeding behavior through actions on the LH, we examined feeding following unilateral injections of bicuculline into the VPm made either ipsilateral or contralateral to a unilateral excitotoxic lesion of the LH in nondeprived rats. We found that lesions of the LH significantly attenuated feeding induced from the ipsilateral VPm, as compared to sham-operated controls. In striking contrast, unilateral LH lesions significantly potentiated the feeding response elicited by injections of bicuculline into the contralateral VPm. The 'ipsilateral-contralateral disruption' design we used makes it extremely unlikely that our findings could have resulted from nonspecific effects of the lesions. These results suggest that the LH is causally involved in mediating the ingestive effects produced by activation of the VPm, and provide an important insight into the functional circuitry by which basal forebrain structures control food intake in mammals.

Injections of muscimol into the paraventricular thalamic nucleus, but not mediodorsal thalamic nuclei, induce feeding in rats

The paraventricular thalamic nucleus (PVT) is a component of the midline thalamic group that is interconnected with several brain regions known to play important roles in the control of food intake, including the lateral hypothalamus and nucleus accumbens shell, suggesting that the PVT itself may be involved in mediating feeding behavior. In the current study, we examined whether inhibition of cells in the PVT with the GABA(A) agonist muscimol could alter food intake in non-deprived rats. To control for possible spread of the drug, we also observed food intake after injections of muscimol into the overlying ventricle or laterally adjacent mediodorsal thalamic nuclei (MD). We found that muscimol injections into the central PVT dose-dependently increased food intake. In contrast, intra-MD injections of muscimol resulted in a potent dose-dependent suppression of food intake, while those into the overlying ventricle had no effect. These results support the proposal that the PVT is a component of the neural circuitry controlling feeding behavior.

Effects of muscimol in the nucleus accumbens shell on salt appetite and sucrose intake: a microstructural study with a comment on the sensitization of salt intake

Previous work has demonstrated that injections of the γ-aminobutyric acidA (GABAA) agonist muscimol into the nucleus accumbens shell (AcbSh) induce pronounced increases in the intake of solid foods and sucrose solutions, but do not potentiate water intake. In order to clarify the range of situations in which inactivation of the AcbSh potentiates ingestive behavior, we examined the effects of muscimol injections on the intake of a 3% NaCl solution in sodium-depleted animals. Although sodium-depleted subjects avidly consumed this solution, muscimol injections had no effect either on the volume consumed or on a variety of microstructural licking parameters. In contrast, in these same animals, muscimol injections significantly increased licking of a 10% sucrose solution. These results suggest that inactivation of the AcbSh may selectively increase the intake of foods, but not that of other homeostatically relevant ingestates. Examination of microstructural parameters suggested that the effect of muscimol on sucrose intake was not mediated by alterations in the "palatability" of the sucrose solution. We also observed that sodium-depleted subjects displayed significantly larger salt intakes after their second experience with sodium depletion than their first, and microstructural analysis in this case indicated that this sensitization effect was produced in a manner consistent with the animals showing increased "hedonic responsiveness" to the salt solution.

GABA(A) and dopamine receptors in the nucleus accumbens shell differentially influence performance of a water-reinforced progressive ratio task

Several authors have shown that injections of the GABA(A) agonist muscimol into the medial shell region of the nucleus accumbens (AcbSh) result in large increases in food, but not water, intake. In previous studies we demonstrated that intra-AcbSh injections of either muscimol or of the indirect dopamine agonist amphetamine increase response output on a food-reinforced progressive ratio schedule. In the current experiment we extended these observations by examining the effects of muscimol and amphetamine injections on the performance of a water-reinforced progressive ratio task in mildly deprived animals. We found that muscimol did not affect the number of responses made in the water-reinforced task, even though a marked increase in responding was observed after amphetamine. Muscimol did, however, significantly increase food intake in the same animals. The results suggest that the enhancing effects of intra-AcbSh muscimol differ from those of amphetamine in that they are selective for food-reinforced behaviors.

Inactivation of the median raphe nucleus increases intake of sucrose solutions: a microstructural analysis

Previous studies have shown that microinjections of the GABA-A agonist muscimol into the median raphe nucleus (MR) result in large increases in the intake of solid foods. In the current study, we used microstructural techniques to characterize the effects of intra-MR muscimol injections on the consumption of either a 0.05 M or a 0.29 M sucrose solution. After injections of either saline or muscimol, animals consumed more of the 0.29 M than the 0.05 M solution, an effect which resulted primarily from increases in the initial rate of consumption with no change in the rate at which licking decayed across the test session. In contrast, intra-MR muscimol injections had little effect on the initial licking rate, but greatly increased meal duration, indicating that this treatment affected ingestion in a different way than did altering the sucrose concentration. Muscimol injections produced a significantly larger increase in the intake of the 0.29 M than of the 0.05 M solution. Intra-MR muscimol injections did not alter the within burst rate of licking, suggesting that they did not affect the functioning of the licking pattern generator. In contrast, these injections did increase the number of licks contained within "clusters," that is groups of licks separated from each other by intervals of more than 0.5 sec. These findings show that inactivation of the MR produces a powerful effect on the intake of liquid diets, and that the nature of this effect is different from that produced here by changes in sucrose concentration and from those reported after pharmacological manipulations of a number of other brain systems. We additionally discuss several theoretical issues arising in the interpretation of microstructural data. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Evidence that the nucleus accumbens shell, ventral pallidum, and lateral hypothalamus are components of a lateralized feeding circuit

Pronounced feeding can be elicited by injections of the GABA(A) agonist muscimol into the medial shell region of the nucleus accumbens (AcbSh). This region of AcbSh has been shown to project to both the lateral hypothalamus (LH) and the medial ventral pallidum (VPm). The current study examined the effects of unilateral LH or VPm lesions on the ingestive responses induced by injections of muscimol into the AcbSh on either the same or the opposite side of the brain. We found that lesions of either of these structures drastically attenuated feeding induced from the ipsilateral, as compared to the contralateral, AcbSh. The "ipsilateral/contralateral disruption design" employed here virtually rules out the possibility that the suppressive effects of the lesions were nonspecific and suggests that the VPm and LH play essential roles in mediating the ingestive effects of inactivation of the AcbSh.

Opposite effects on the ingestion of ethanol and sucrose solutions after injections of muscimol into the nucleus accumbens shell

Injection of the GABA(A) receptor agonist muscimol into the nucleus accumbens shell (AcbSh) elicits robust feeding in satiated rats, but has no effect on water intake. The current study was designed to examine whether intra-AcbSh muscimol injections influence the intake of ethanol solutions in rats trained to drink using a limited access paradigm. We confirmed that bilateral injections of muscimol (100 ng) into the AcbSh produce large increases in the intake of sucrose solutions and of the chow maintenance diet but found in two independent experiments that these injections potently reduce the intake of a 10% ethanol solution. Furthermore, intra-AcbSh muscimol significantly increased intake of an ethanol-sucrose mixture. These results demonstrate that activating GABA(A) receptors in the vicinity of the AcbSh can have opposite effects on the intake of different caloric substances and are consistent with the possibility that GABAergic circuits in the AcbSh may play a role in mediating voluntary ethanol intake.

Evidence for motivational effects elicited by activation of GABA-A or dopamine receptors in the nucleus accumbens shell

Microinjections of the inhibitory GABA-A receptor agonist muscimol into the shell region of the nucleus accumbens (AcbSh) have been reported to induce large increases in food intake, but the effect of these injections on motivational processes is less clear. In the current study, bilateral injections of saline, muscimol (50ng/side) or d-amphetamine (10mug/side) were made into the AcbSh of rats trained to lever press on a progressive ratio schedule for food reward. Injections of both muscimol and amphetamine were found to produce a large increase in the breaking point relative to saline injections. This result suggests that inactivation of the AcbSh does not simply drive ingestive behavior, but also affects motivational processes assessed by the progressive ratio schedule. Breaking points were also increased by injections of amphetamine into the AcbSh.

Activation of feeding-related neural circuitry after unilateral injections of muscimol into the nucleus accumbens shell

Chemical inhibition of neurons in the nucleus accumbens shell (AcbSh) elicits intense, behaviorally specific, feeding in satiated rats. We have demonstrated previously that this treatment activates a number of brain regions, most significantly the lateral hypothalamus (LH). This activation could be elicited through a direct neural connection with the AcbSh or secondarily through changes in autonomic activity, stress, or circulating levels of orexigenic or satiety factors. In the present study, we used the immunohistochemical localization of Fos protein to map neuronal activation after unilateral muscimol injections into the AcbSh to determine whether AcbSh-mediated Fos expression remains lateralized in the circuit and whether secondary systemic changes in the rat can be excluded as primary factors in the activation of downstream component nuclei. Rats receiving only saline injections exhibited very little Fos immunoreactivity. In contrast, unilateral injections of muscimol into the AcbSh consistently increased Fos expression in several brain regions. Three distinct patterns of expression were observed. Fos synthesis in the LH was increased only on the side of the brain ipsilateral to the muscimol injection. Fos expression remained primarily ipsilateral to the injection site in the septohypothalamic, paraventricular hypothalamic (PVN), paratenial thalamic, and lateral habenular nuclei, and medial substantia nigra, but was increased bilaterally in the piriform cortex, supraoptic nucleus, central nucleus of the amygdala, and nucleus of the solitary tract. Smaller numbers of Fos-immunoreactive cells were seen unilaterally in the bed nucleus of the stria terminalis, medial ventral pallidum, arcuate nucleus, and ventral tegmental area and bilaterally in the supraoptic and tuberomammillary nuclei. The labeling in the LH, PVN, and other unilaterally labeled structures provides evidence that these brain regions are components of an AcbSh-mediated neural circuit and suggests that they may be involved in the expression of AcbSh-mediated feeding behavior.

NPY mediates the feeding elicited by muscimol injections into the nucleus accumbens shell

Injections of muscimol into the nucleus accumbens shell (AcbSh) induce large increases in food intake in satiated rats and also activate neurons in a number of feeding-related brain regions, including NPY-containing neurons in the arcuate hypothalamic nucleus and cells in the paraventricular hypothalamic nucleus. This suggests that the NPY system may participate in the expression of AcbSh-mediated feeding behavior. Therefore, we examined the effects of intraventricular administration of the Y1 receptor antagonist 1229U91 or the Y5 receptor antagonist L-152,804 on AcbSh-mediated food intake. Intra-AcbSh muscimol elicited a large increase in food intake which was potently suppressed by blocking either central Y1 or Y5 receptors. Our results suggest that the AcbSh influences food intake, in part, through the release of NPY.

Forebrain lesions differentially affect drinking elicited by dipsogenic challenges and injections of muscimol into the median raphe nucleus

Injections of muscimol into the median raphe nucleus (MR) elicit intense drinking in normally hydrated rats. To determine whether this response is dependent on forebrain systems mediating other aspects of water intake, the authors examined the effects of lesions of the subfornical organ (SFO), median preoptic nucleus (MnPO), lateral preoptic area (LPO), or lateral hypothalamus (LH) on the drinking. Lesions of the SFO or LH attenuated muscimol-elicited drinking, whereas lesions of the MnPO or LPO increased water intake after the treatment. All of the lesion groups showed a deficit in drinking to injections of polyethylene glycol and at least one of the doses of hypertonic saline. Only the SFO- and LH-lesioned groups showed a suppression of drinking to systemic injections of angiotensin II, suggesting that the drinking elicited by intra-MR injections of muscimol may involve changes in the central circuits mediating angiotensin-induced drinking.

Evidence of a functional relationship between the nucleus accumbens shell and lateral hypothalamus subserving the control of feeding behavior

Inhibition of neurons in the nucleus accumbens shell (AcbSh) with local injections of GABA agonists or glutamate antagonists elicits an intense, but specific, feeding response resembling that seen after stimulation of the lateral hypothalamus (LH). To help characterize the contribution of the LH to the expression of AcbSh-mediated feeding, we used the immunohistochemical detection of the nuclear protein Fos to determine whether inhibition of AcbSh cells results in an activation of LH neurons. Injections of the GABA(A) agonist muscimol into the AcbSh greatly increased the number of cells exhibiting Fos-like immunoreactivity in the LH, as well as in the lateral septum, paraventricular hypothalamic nucleus, ventral tegmental area, substantia nigra pars compacta, and nucleus of the solitary tract. Blocking activation of LH neurons with the selective NMDA receptor blocker D(-)-AP-5 is known to suppress deprivation-induced feeding. We found that injections of D(-)-AP5 into the LH also dose-dependently suppressed AcbSh-mediated feeding. It is likely that inhibition of GABAergic neurons in the AcbSh is responsible for eliciting this feeding. If a behaviorally relevant GABAergic projection terminates in the LH, we should be able to mimic the effects seen after inhibition of the projection neurons by applying a GABA receptor blocker to the area. However, injections of the GABA(A) receptor blocker bicuculline or the GABA(B) receptor blocker saclofen did not significantly affect food intake. Thus, it appears that the expression of the feeding response depends on an NMDA-preferring receptor-mediated activation of LH neurons and is not the result of disinhibiting LH cells by disrupting transmission at GABA synapses.

Blockade of GABAA receptors in the medial ventral pallidum elicits feeding in satiated rats

gamma-aminobutyric acid (GABA)-containing fibers from the nucleus accumbens shell (AcbSh) terminate in the medial ventral pallidum (VPm) and neurons in the VPm project to the lateral hypothalamus (LH). Therefore, the VPm is anatomically interposed between the AcbSh and LH, two functionally related brain regions that mediate food intake. The present study demonstrates that blockade of GABAA receptors in the VPm by local administration of bicuculline greatly increases food intake in satiated rats. The data suggest that an AcbSh-VPm-LH circuit may be involved in the control of feeding behavior.

Specific changes in food intake elicited by blockade or activation of glutamate receptors in the nucleus accumbens shell

Blockade of non-N-methyl-D-aspartic acid (NMDA) ionotropic glutamate receptors in the nucleus accumbens shell (AcbSh) with 6,7-dinitroquinoxaline-2,3-dione (DNQX) elicits intense feeding in satiated rats. In order to determine whether or not this feeding is part of a general behavioral activation, we observed the effect of intra-AcbSh DNQX injections on intake of solid food, liquid food, and water, and on gnawing behavior. In addition, we investigated the possibility that activation of a subset of these receptors with (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) could suppress feeding. DNQX significantly increased intake of solid and liquid food, but did not significantly affect water intake or gnawing behavior. Furthermore, injections of AMPA into the AcbSh suppressed deprivation-induced feeding and intake of a palatable 5% sucrose solution without affecting water intake in water-deprived rats. Taken together, these data suggest that DNQX is acting on a system specifically involved with the regulation of food intake.

Placement in a novel environment induces fos-like immunoreactivity in supramammillary cells projecting to the hippocampus and midbrain

Injections of fluorescent retrograde tracers into either the hippocampal formation or the midbrain raphe nuclei resulted in retrograde labeling of many cells in the supramammillary region of the hypothalamus. Double labeling studies indicated that these two projections originate from different populations of supramammillary cells. Expression of the proto-oncoprotein Fos could be induced in some retrogradely labeled cells by placing rats in a novel open field before sacrifice. Although seen in both cell types, Fos-like immunoreactivity was significantly more common in supramammillary cells projecting to the hippocampus than in those projecting to the midbrain. These findings suggest that the supramammillary region may contain several populations of neurons which are differentially responsive to certain behavioral manipulations.

Differential behavioral responses to dopaminergic stimulation of nucleus accumbens subregions in the rat

The following experiments investigated the behavioral response to local microinfusion of dopamine (DA) and selective DA agonists into the core and shell subregions of the nucleus accumbens. Rats were implanted with chronic indwelling cannulae aimed at these subregions. Two experiments were conducted. In experiment 1, the response to DA (0, 2, 5, 10 microg/0.5 microl/side), the D-1 agonist SKF-82598 (0, 0.1, 1.0 microg), the D-2/3 agonist quinpirole (0, 1, 5, 15 microg) and the D-3 preferring agonist pramipexole (0.1, 1.0, 10.0 microg) was examined in photocell activity cages. Locomotor (horizontal) and rearing (vertical) activities were measured. DA and SKF-82958 induced relatively greater increases in activity following stimulation of the shell as compared with the core. Quinpirole induced a dose-dependent suppression of activity after infusion into both sites, although the core was more sensitive to the suppressive effect than the shell. Pramipexole induced time-dependent, biphasic effects that were small in magnitude and did not differentiate between site. In experiment 2, an observation procedure was used to record behaviors (locomotion, rearing, feeding, drinking). Dopamine (0, 2, 10 microg) elicited greater increases in rearing and feeding behavior in the shell than in the core. SKF-82958 (0, 0.75 microg) enhanced locomotion and rearing to a similar extent in both subregions in this test, whereas a mixture of a low dose (0.25 microg) of the D-1 and D-2 agonists selectively induced behavioral activation in the shell. In contrast to the results in the activity cage test, quinpirole (0, 1, 5 microg) increased motor activity at the lower dose when infused into the shell but not into the core. No alterations in feeding were observed following infusion of selective agonists, and no changes in drinking were found with any of the treatments. In summary, the shell appears to be relatively more sensitive to the motor activating effects of DA agonists than the core. Moreover, circuits associated with shell may be preferentially involved in feeding.

GABA in the nucleus accumbens shell participates in the central regulation of feeding behavior

We have demonstrated previously that injections of 6, 7-dinitroquinoxaline-2,3-dione into the nucleus accumbens shell (AcbSh) elicits pronounced feeding in satiated rats. This glutamate antagonist blocks AMPA and kainate receptors and most likely increases food intake by disrupting a tonic excitatory input to the AcbSh, thus decreasing the firing rate of a population of local neurons. Because the application of GABA agonists also decreases neuronal activity, we hypothesized that administration of GABA agonists into the AcbSh would stimulate feeding in satiated rats. We found that acute inhibition of cells in the AcbSh via administration of the GABAA receptor agonist muscimol or the GABAB receptor agonist baclofen elicited intense, dose-related feeding without altering water intake. Muscimol-induced feeding was blocked by coadministration of the selective GABAA receptor blocker bicuculline, but not by the GABAB receptor blocker saclofen. Conversely, baclofen-induced feeding was blocked by coadministration of saclofen, but was not affected by bicuculline. Furthermore, we found that increasing local levels of GABA by administration of a selective GABA-transaminase inhibitor, gamma-vinyl-GABA, elicited robust feeding in satiated rats, suggesting a physiological role for endogenous AcbSh GABA in the control of feeding. A mapping study showed that although some feeding can be elicited by muscimol injections near the lateral ventricles, the ventromedial AcbSh is the most sensitive site for eliciting feeding. These findings demonstrate that manipulation of GABA-sensitive cells in the AcbSh can have a pronounced, but specific, effect on feeding behavior in rats. They also constitute the initial description of a novel and potentially important component of the central mechanisms controlling food intake.

Injections of nociceptin into nucleus accumbens shell or ventromedial hypothalamic nucleus increase food intake

The novel opioid receptor ORL1 is widely distributed throughout the CNS of the rat, and is present in high densities in several brain regions known to participate in the control of food intake. We injected a recently identified endogenous agonist of this receptor, nociceptin, into two of these feeding-related areas. Microinjections of nociceptin (2.5-25 nmol) into either the ventromedial hypothalamic nucleus or the nucleus accumbens shell significantly increased food intake in rats. We believe this to be the first report of a specific effect of nociceptin on a motivated behavior.

Simultaneous administration of neuromedin B-10 and gastrin-releasing peptide(1-27) reproduces the satiating and microstructural effects of bombesin

Bombesin (BN) suppresses food intake more than an equimolar dose of neuromedin B-10 (NMB-10) or gastrin-releasing peptide(1-27) [GRP(1-27)]. Because BN is active at both NMB- and GRP-preferring receptors, it is possible that its greater potency is due to the simultaneous activation of both receptor subtypes. This hypothesis was tested by administering a mixture of NMB-10 and GRP(1-27). The results were consistent with our hypothesis: administration of the mixture of NMB-10 and GRP(1-27) produced an inhibition of intake volume and total licks equivalent to that seen after an equimolar dose of BN and resulted in similar changes in the rate and pattern of licking.

Fourth ventricular injection of the bombesin receptor antagonist [D-Phe6]bombesin(6-13)methyl ester, but not BW2258U89, increases food intake in rats

To investigate the role of endogenous bombesin-like peptides in the caudal brainstem for the short-term control of food intake, we evaluated the effects of fourth-ventricular injections of two different bombesin (BN) receptor antagonists, [D-Phe6]BN(6-13) methyl ester and BW2258U89, on intake of sweetened, condensed milk in male rats. Although fourth-ventricular administration of BW2258U89 (0.125-20 ng) had no effect on food intake, fourth-ventricular injections of 1.0-20.0 ng of [D-Phe6BN(6-13) methyl ester resulted in an inverted U-shaped, dose-response curve with a maximal effect at 2.5 ng. Microstructural analysis of the licking behavior indicated that the increase in intake was primarily the result of an increased number of licks and an increase in lick efficiency. Behavioral time sampling demonstrated that these changes in intake occurred without the appearance of any competing behavior or significant change in the overall pattern of behavior. Because [D-Phe6BN(6-13) methyl ester appears to be a preferential antagonist at the GRP-preferring receptor, the increased intake that occurred after its administration suggests that an endogenous GRP-mechanism in the caudal brainstem is necessary for the normal, short-term control of sweet milk intake under these conditions.

Microstructural analysis of licking behavior following peripheral administration of bombesin or gastrin-releasing peptide

To determine the effects of bombesin (BN) and gastrin-releasing peptide [GRP(1-27)] on food intake and the microstructure of licking, the rate and pattern of licking were measured by lickometer after intraperitoneal administration of 2, 4, 8, and 16 micrograms/kg of each peptide. Although both peptides inhibited intake significantly, the dose-response functions of the peptides were different. At lower doses (2-4 micrograms/kg) the peptides were equipotent; at higher doses (8-16 micrograms/kg) BN was significantly more potent than GRP(1-27), possibly due to an action of BN on neuromedin B (NMB) receptors. The microstructural results demonstrated that both peptides decreased intake by increasing the rate of decay of licking and decreasing the number of bursts and clusters without changing the initial rate of licking or the size of the bursts and clusters. This combination of microstructural changes is evidence that both peptides increased the postingestive negative feedback effects of the diet without changing the orosensory positive feedback. These results are consistent with, but do not prove, the hypothesis that endogenous GRP(1-27) acts as a postingestive satiety signal in rats.

Studies on the behavioral activation produced by stimulation of GABAB receptors in the median raphe nucleus

Injections of the GABAB agonist baclofen into the median raphe nucleus (MR) resulted in marked hyperactivity and in increases in food and water intake by non-deprived animals. The locomotor effects of baclofen were stereospecific and could be antagonized by coinjection of the GABAB antagonist 2-hydroxysaclofen. Hyperactivity was produced by lower doses of baclofen, at shorter latencies, when the drug was injected into the MR than when it applied to the dorsal raphe nucleus (DR) or the ventral tegmental area (VTA). The locomotor response to intra-MR baclofen was unaltered in animals pretreated with the serotonin synthesis inhibitor p-chlorophenylalanine. Finally, intra-MR injections of baclofen produced a large increase in dopamine metabolism in the nucleus accumbens and striatum but failed to alter hippocampal or striatal serotonin metabolism. These findings suggest that baclofen may produce increases in activity and ingestive behavior as a result of an action on non-serotonergic cells in the MR.

Evidence for GABAergic projections from the tegmental nuclei of Gudden to the mammillary body in the rat

Immunochemical studies have demonstrated the presence of large numbers of cells immunoreactive for glutamic acid decarboxylase (GAD) within the dorsal and ventral tegmental nuclei of Gudden. Following injections of Fluoro-Gold into the medial mammillary nucleus, a substantial proportion of the retrogradely labeled neurons within the ventral tegmental nucleus displayed GAD-like immunoreactivity. Conversely, electrolytic or excitotoxic lesions of the ventral tegmental nucleus produced a large decrease in the number of fibers and terminals immunoreactive for GAD within the medial mammillary nucleus. In contrast, electrolytic lesions of the dorsal tegmental nucleus were found to produce a large decrease in GAD-like immunoreactivity which was restricted to the lateral mammillary nucleus. Control lesions placed caudal to the dorsal tegmental nucleus were without effect. These findings suggest that the dorsal and ventral nuclei send a substantial, topographically organized, GABAeregic input to the mammillary body.

Ascending dopaminergic projections from the dorsal raphe nucleus in the rat

The projections of putative dopamine containing cells within the dorsal raphe nucleus (DR) were studied using a combination of tyrosine hydroxylase (TH) immunocytochemistry and fluorescent retrograde tracing. Substantial numbers of TH-immunoreactive cells in the DR were found to project to the nucleus accumbens. Progressively smaller numbers of cells were found to project to the lateral septum and medial prefrontal cortex. Very few TH-immunoreactive cells projected to the dorsal striatum, and none to the substantia nigra. TH-immunoreactive cells did not display serotonin-like immunoreactivity. These findings indicate that the projection pattern of TH-immunoreactive cells within the dorsal raphe more closely resembles that of dopaminergic cells within the ventral tegmental area (VTA) than that of serotonergic cells within the DR.

Evidence for a projection from the B9 serotonergic cell group to the median raphe nucleus

Following injections of several fluorescent retrograde tracers into the median raphe nucleus large numbers of retrogradely labeled cells were observed in the ventral mesencephalic tegmentum. Subsequent immunocytochemical processing for serotonin-like immunoreactivity suggested that a large component of this projection originates from serotonergic B9 cells. Although tracer injections into the dorsal raphe did not result in similar labeling of B9 cells, evidence was obtained suggesting the existence of a small serotonergic projection from the median to the dorsal raphe.

Evidence that serotonergic projections to the substantia nigra in the rat arise in the dorsal, but not the median, raphe nucleus

Following microinjections of the fluorescent retrograde tracer Fast blue into the substantia nigra, large numbers of retrogradely labeled cells were observed in the dorsal raphe nucleus. Using immunocytochemical techniques it could be demonstrated that the majority of these cells contained serotonin-like-immunoreactivity. In contrast, careful examination of the region of the median raphe nucleus revealed no suggestion of a significant projection from the median raphe to the substantia nigra.